I'll try to explain this bug me and have come across as best as I can, since it seems to be very finicky.

I can't add his pet Musculo to my pet Conga's list of friends. I get an error that says the following.

I added an entirely different pet, Moomba's Grey Blue to Conga's friends without any issues, so it seems to affect very specific pets. He was able to add my pet Auko to his Blue's list of friends, but Auko can't be added to Musculo's friends.

This error also seems to now be vice-versa between both Musculo and Conga. Initially, Moomba was able to add Conga to Musculo's friends a few days ago, but wanted to change the wording. Now he can't add her back.

I'm using Chrome, I'm pretty sure Moomba is using chrome as well.

Things seem to be okay on my end, as far as adding pets to Conga's friends. I'm not sure how things are going for my friend though, I'll have to ask him when I see him online again.

Not yet, I haven't been able to catch him online for some time. :c

No, sorry. D: The glitch does seem to be fixed on my end at least, so you can go ahead and lock this thread. Thank you!

maybe this is a silly question, but are you including apostrophes in the comments for the pet friend (the text box that says 'information')? i've heard of people being unable to add pet friends when they use apostrophes, but it works fine otherwise.

I'm not sure if I was at the time I initially had this glitch, but I tried that now and things worked fine.