She needs proper advice from someone who knows all the circumstances as it makes a difference and no-one can advise fully unless they are properly informed. As Twenty says, some of the advice on here is plain wrong.
Seriously, she is paying the price now for not doing things properly in the first place, by continuing to do things outside of the correct legal procedure she may end up a lot worse off than she is now.
It might take longer than she wants to get it sorted but the possible repercussions if she doesn't do things properly could cause problems for longer still.
I'm not denying her intended course of action might work in this instance, however, with a tenant who seems very up on their legal rights (correctly or not, I can't say) she could be playing with fire and end up with an unlawful eviction case on her hands and potential further problems if she is effectively harassing the tenants generally eg turning the heating off.
It also doesn't put her in a great position for trying get her tenants to abide by the law if she is only too willing to take matters into her own hands. It is a criminal offence to evict tenants other than by court action.
At £120pw (£480 per month) he is effectively just over 6 weeks in arrears so just over a month - not significant in the general scheme of things (especially if she has been witholding things like heat!).
For example, if certain criteria fit, in the rent arrears ground (s.8 notice) I believe that 2 months/8 weeks arrears are needed at the expiry of a properly served s.8 notice (2 weeks) to go for a possession order so a clever tenant could pay just what is needed without clearing the arrears. Issues like disrepair can also come into play here. She needs proper advice to deal with various scenarios.
I'm guessing from the way she has done things that she has little idea as to the legislation and local authority p