Do not press it too much. The few times a friend or family member has noticed my lack of hair I ended up getting really anxious and flustered and ended up making excuses.

I would just sorta pretend you don't notice anything and continue to be a good friend. Pulling out hair doesn't really cause physical harm, unless she is actually swallowing her hair once it is pulled out. Let her come to you with the problem if she wants to. Otherwise be supportive without pointing out something that makes trichotillomania sufferers extremely nervous.