Who keeps telling you ?
I mean, owning a whole property is obviously better than owing part of one, but, if you / he could afford that, then you wouldn't be looking at SO.
Personally, I think paying rent is dead money.
Yes, you have to factor in buying and selling costs, if you are thinking you might be moving on in a couple of years, but if - as you seem to suggest - that is unlikely, then I'd have thought starting to buy would be the sensible thing to do.
I think, should he be wanting to sell at a future time when it is a buyers market, it would be harder to sell than a non-SO property, but there will always be buyers, in the same way that he is considering it now.
Obviously, he needs to do the maths, about paying rent and a small mortgage, but I'm presuming he has done that.
Might also be worth him talking to other tenants to see what they have found to be good, or bad about the landlords of the other 75% of the property.