Well if this is a constant thing where your boyfriend is constantly putting up money or placing himself in danger for this person then I might well say something.

However as a mother anD someone who has many friends with kids if they asked this of me then absolutely I would try to help. I have a long distance n, I visit one day a month. But it's no skin off my nose if we spend one day putting a friends mind at ease. There is always next month.
If the kid had ended up in hospital or o'd'd I would never forgive myself.
Nor would I feel comfortable imposing my opinion on my partner about his friend.
It seems perfectly practical to ask for assistance of someone local to the area. Yes he could have driven through, and in that time the person living their could have found him.
Why didn't he pick him up and take him home.
Possibly Because the relationship between addicts and their families are complicated tenuous and prone to massive fights and ruptures. I doubt the father would have received as positive reaction as your partner did when he showed up.
It sounds like your partner values his friendships and has empathy for his friend in this situation. Those are hard things to come by in anyone so I find it difficult to find fault.
I assume you don't believe your partner would put you are any actual risk of being hurt, so it's doubtful he felt this kid was a threat.

I suppose it just seems a little odd to me that if this is a one off event why it irks you so much.

no one was harmed.
Your partner feels good about himself.
His friend is grateful.
The kid is safe.
And whilst it wasn't what you expected you still spent the day with your fella.

Maybe I am missing something. I do hope this isn't something that effects your relationship, hopefully next visit will be more to your liking.
All the best and Take care.