agree she should pay rent- but take legal advice to make sure it’s seen as a rent payment and also provides her with some protection so she can’t immediately be kicked out.
sounds like they are young and in the early stages of a relationship.
People have said he is profiting from a partner charging her rent but she is also profiting from not paying rent (which she currently is)
if he were to buy it and live separately vs having his gf live there rent free, he would be in exactly the same situation if they broke up in 5 years time. The gf, however, would be 5 years of rent better off if they split up in 5 years time and she had saved it.
a fairer way would be to halve the amount of rent she is currently paying e.g she currently pays £600 per month and pays your son £300 per month and then saves £300 per month in her own name. This means they are equally the same amount better off should they split up in the future.
the other comments are red herrings
e.g if they split up she would have contributed to someone else’s mortgage and got nothing- same as renting
romantic partner not a lodger- but it doesn’t sound like they are at the point of being serious enough if they have already broken up in the last year
alot of people still think it’s the 50s and people should get married before living together or living together automatically means you are ready to get married/serious.
I say this as someone who has been in this position and also have a daughter.
most important thing is to take legal advice to confirm the rent.