Did your friend ask T to come? Or did T wanted to see this play? i.e. Is the T there as T or as "normal" human being?

If both sides are OK with it (and discussed the reasons behind it to make sure they are not missing stg) I don't see it as boundary crossing. Though it would be for me- no way I'd like to meet my T outside of therapy room. I guess it depends on the ppl involved.