I don't see it crossing a line, but I can see why a therapist might be cautious about not wanting it to end up causing more harm in some way. My therapist is pretty conservative in this regard, but that could be because I can't handle too much of that stuff. The most she has said is that she's always glad to see me, which is probably the equivalent of an I love you to my ears. I don't know if she's more gushy with others or if that's the extent of her emotional range with clients.

If it's the therapist feels that way though, and the person wants to hear it, I don't see why they shouldn't say it.