I really wouldn't recommend that. You'd be looking at a selection from complete stonewalling, aggressive defence/denial/accusations of blackmail, malice and insanity, emotive pleas about 'but it's for the children', 'you'd destroy this school and the hardworking, dedicated teachers and TAs, as they'd lose their jobs' and 'you'd destroy me'. And, if you decide to ignore this and go to the Head, don't breathe a single word of the TA friend's comments - she'll be straight up for dismissal in retaliation/an attempt to get rid of a threat/accusations of being the one who actually cheated and therefore potentially not being able to ever get another job in a school.
When it comes down to it, cheating in SATs is for the benefit of the Head. It harms the children whether they have unmet SEND, a potential to attain highly at GCSE or absolutely normal, average ability, as they are not getting the mark that represents their true level. It hides inadequate management and teaching. It negatively affects their secondary schools. It also provides leverage for the Head to enjoy considerable financial advance from payrises and future job opportunities. If unreported, it furthers a position of entitlement and untouchability, which could also be replicated in how they deal with staff and pupils alike in terms of employment law, SEND law, financial regulations and health and safety. It breaches the Nolan principles for behaviour in Public Life. And, most importantly, it's wrong.
The PP who says it's nothing because they're an independent school Head would feel differently if it were their entrance tests being altered, whether so that children who didn't meet their requirements for entry obtained places or whether that cheating made a child eligible for a bursary/scholarship at the expense of another child whose assessment was completed fairly and honestly.
If this is in any way a real post - and there's no actual reason to suppose it isn't, as there are cases every year in every area and type of school from the leafiest suburb to the inner city - then the only appropriate and safe way to proceed is to report it externally. There would then be an investigation that doesn't put vulnerable, low paid staff at risk of repercussions (somebody on a temporary, year to year contract at less than NMW over the year due to being TTO and desperate to keep employment is in a very weak position compared to a confident Pillar of the Community who can decide unilaterally to not renew their contract for the next academic year, for example). It may be that any number of other parents have also been told this happened and will report it, maybe one of those vulnerable members of staff have decided that despite the risk to themselves, they will do the right thing and report the maladministration - additional reports from parents will protect them.
Not having KS2 results at all is less harmful to a kid than having falsified data. You will not harm a single child from reporting this. And it's the right thing to do.