I want to clarify my post: I'm not saying that OP's friend's new church necessarily is a cult, any more than I'm saying it's an abusive boyfriend/husband.
I'm saying that this kind of tactic is one of the many types of controlling and manipulative behaviour that is also commonly used both in high-control organisations like cults, and in some types of abusive relationships. (There are a surprising number of parallels between the two — I guess there are only so many ways to efficiently and effectively manipulate and control people's behaviour.)
But I think it's important to acknowledge that controlling, manipulative, abusive behaviour of this type from organised groups doesn't necessarily come from something most of us would generally consider a cult (e.g. new religious movement/weird spiritual stuff/aliens and doomsday and suicides/sex stuff/drug stuff/extreme financial and personal obligations/fringe movements of established religions with heretical beliefs/a charismatic leader etc.). IMO it's good to be able to recognise it for what it is, wherever you see it.
Lots of mainstream (or smaller but generally moderate) religions have subgroups within them that use some of the control techniques often used by cults (or abusive partners), without necessarily having many or any of the other common features of a cult, and without fundamentally diverging from common beliefs of the broader religion. And they can still be dangerous.
I don't think that the people in the church OP describes are necessarily malicious in what they're doing and saying, even if they are dangerous. (The same for many cult members TBH — obviously many truly believe their cult is the answer, at least for a while, so I can't berate them too much.) I think it's likely that the church members who've done this to OP's friend don't even fully recognise the controlling, manipulative nature of their behaviour.
Where I might see a love-bombing (coined by the Moonie cult) campaign, the nature of which has been refined over decades to maximise effectiveness, they might see a genuine expression of their caring, and of their wish to share their "good news".
They reach out to a person, broken or badly struggling, lonely or lost in life, befriend them, invite them warmly, enthusiastically welcome them, take them to meet new people who will all be lovely to them, envelop them in a community with all kinds of activities and social groups, spend lots of time listening attentively to them and demonstrating they care about them and what they think, give them interesting new concepts to learn about, and tell them that they/their life can be fixed and that they need never be alone or uncertain about anything again, because God (in this case) knows everything, has a plan for you, Jesus loves you more than you can imagine and can take on all your burdens for you, whatever. Probably, they really believe the teachings and want to "save" the person, and don't recognise how they're taking advantage of that person and using powerful techniques on them — or of they do, they reframe it as something like finding people who are at the right point in their life to be receptive to the message. There's probably a Bible story they can point to or something, about how people are ripest for a big religious conversion when life has shat on them from a great height and they're utterly wretched and defenceless.
Where some of us might see vulnerability — a predatory church with extreme beliefs pouncing on people at their weakest moments, when they're least able to resist the love-bombing or the allure of being told they'll be understood and cared for and feel like they belong somewhere and have a purpose — maybe the grassroots church members see someone who NEEDS God, right now, a broken soul who can be SAVED etc. etc. And how can there be anything wrong with being welcoming and telling someone they're loved? Assuming they're basically nice people, their desire to be welcoming, and to help those who are suffering, lines up nicely with one of the most effective techniques for indoctrinating someone into a high-control group. Which church leaders may or may not be more conscious of than some of the followers…
And if those existing church members have also been told by their spiritual leaders that new Christians are most at risk from falling back into sin, maybe even that the devil could speak through their friends and family to draw them away from Christ, or that those young in their faith are not yet strong enough to resist those things of the world that can blind you to the true path and so forth, then it's only natural for them to earnestly advise a new Christian not to risk mortally wounding their fragile new faith, not to continue to expose themselves to non-Christian people, non-Christian media, non-Christian anything.
Except that "You shouldn't spend too much time around people not in our group, or they'll lead you astray" is exactly the same thing as cults or abusers not wanting outsiders being able to influence their victims into breaking free. It's just different words. Lead astray, break free, tomayto etc.
These are dangerous, coercive, abusive behaviours, maybe even more dangerous because it's not coming from an obvious cult — a Christian church, even an extremely socially conservative charismatic or evangelical one, usually still has a sort of baseline level of acceptability and respectability in the UK — and also because it's possible that most of the people carrying out the actual interpersonal control techniques don't even realise the manipulation and abuse they're perpetrating.
I raised the similarity with the both cult tactics and abusive relationships, not to dismiss this church as a cult (which it may or may not be, depending on your definition, but nothing in the beliefs OP describes is so incredibly unusual for Christianity that it could be considered fringe), but to highlight the behaviour itself as manipulative abuse — even when it superficially doesn't appear abusive (or we've got so used to it from certain types of organisation we've stopped noticing how weird and controlling it is), and even though it's a church and many people generally think of churches in the UK as mostly benign. (There's a fuck of a lot of churches act like this, though. Not the majority by any means, but there's got to be at least one in every town.)