Hi Rosemarie,
I love fitness buddies, they really help when it comes to motivation and maybe this is your chance to motivate them. I would definitely have an honest talk with them on a day/time when you usually don't workout. You can call them up and tell them "Hey, is there something going on that is affecting you? Because lately I've noticed your motivation for working out has really waned"
Maybe there is something in their life that is causing them not to want to be around other people. If you still feel they would rather not talk about it or don't want to continue working out, leave the door open and let them know you are there for them and let them be. Keep working out and try finding someone else who can be another fitness buddy. Maybe in the future when you fitness buddy gets out of their slump, they can join you guys.
All the best
Kenn Kihiu
http://www.dancexfitness.com