Your friend has broken the law and it is going to be a huge ball ache now to get the tenant out. Personally I think this is a good thing as your friend is going to learn a costly lesson so that they don't make this mistake again - no one should be a landlord unless they understand their responsibilities.
Firstly, your friend cannot issue a Section 21 eviction as it won't be valid.
They can issue a Section 8 eviction on the grounds of rent arrears but it'll only be valid if the tenant has reached the trigger points for arrears (two months arrears for monthly paid, eight weeks for weekly paid, three months if they pay quarterly). If the tenant pays anything that reduces their rent arrears below the trigger point, even by just £1, then the Section 8 is no longer valid.
Basically as soon as your friend goes in front of a judge to try remove the tenant they are going to be fucked because a judge has the power to award between 100% and 300% of the deposit in compensation to the tenant on top of the deposit itself, so 4x whatever the amount it is. There are no win-no fee solicitors who will help the tenant with this and any costs will be recovered from your friend if/when they lose - the majority of tenants win these claims.
Your friend needs to speak to a solicitor about how to proceed with minimal financial loss. It might also be best to try and negotiate with the tenant if they are usually a good tenant who pays on time and this non-payment is just down to losing their job, see if a payment plan can be worked out to recover the loss or if they've applied for benefits to help with rent, etc.
And for the love of God, put the deposit in a scheme now. If/when it goes before a judge they will look more favourably on a (very) late protection than no protection at all.