"Things turning nasty' really makes some suggestions difficult but, the parents must recover the house. I agree with Lance. I'd go for the leagl options but when?...before or after trying things. Was the 'nastiness' serious enough for an avo?.Was it serious enough to prevent him returning?
Is there some legal reason he cannot return for his gear when she is absent? or is he still as silly as he was to have her there in the first place.
They might also, being proprietors, change the locks for locks with keys locksmiths will not copy or open. I would not allow her to re-enter the premises but on an agreed day giving reasonable notice, made in writing or served on her,put her gear out, just inside the front gate, preferably under plastic,for collection. Another more I'd speak with a solicitor and if advised practicable, leave her notice that she can negotiate with the solicitor a short lease with bond in which she agrees to vacate the premises on a certain date or agrees to be removed. I doubt in this case any lease can be inferred as she was living only as companion to the son. He may have been nasty or decent and she may have been hell to live with or a persecuted woman but the fact is he has gone and she is still there. The age of the daughter may end up as a relevance in a legal contest. The problem is that getting resistant occupiers out can be time taking and costly. The other problem is to bear in mind the reaction of the tribunal looking at all the facts assembled and in all things be guided by good legal advice, which may include an independent hearing.