they assumed he had a great time so it wasn't an intentional thing.
That's the thing though, I think those who invite a friend with a "surrogate sibling" intention (not all do, obviously) are only considering the needs of their child. Your child is the means to the enjoyment, but their feelings/considerations are not taken into account.
but I still find the level of emotional investment from her a little odd.
Yes! In our case the mother was extremely intense, phoning constantly to try to arrange meetups etc and once I had 2 missed calls from her then an email entitled "WHY ARE YOU IGNORING ME????!!!!" She then started telling me that she noticed my dd seemed most happy when she was with her dd, so that meant that we should meet up at least 3 times per week She would highly praise my dd, pointing out really positive qualities etc, whilst her dd was really manipulative and horrible to mine.
It developed into a really narc situation, where she would make out that all the meetups were actually for my benefit. Something happened and I gently confronted her, she then latched on to someone else and started slagging my dd off, whilst highly praising the "new" friend. It was just a very repetitive cycle that I'm glad I'm out of.