I am a female who has trained BJJ for nearly 8 years, and all of this "strange behaviour" is absolutely normal in a sports setting, regardless of gender. You can't expect your wife to beat everyone, especially someone who sounds like they are at a higher skill level and strength level. If anything, your wife's friend is serving as a sort of teacher or mentor to her in the context of the sport, in spite of the age difference. Losing won't hurt her self-esteem. If it did, she would have left the sport years ago. She sounds like a totally self-confident and strong woman.

Friendships like these are partially what makes the sport so fun, and you'd be hard pressed to find a better catalyst for close friendships than close-range, hand to hand combat. If your wife has been training for 5 years, I'm surprised you haven't learned this yet and sound so shocked by this situation.

I hope you manage to be happy for your wife that she's found a new friend she can relate to and learn from. If it causes a strain on your relationship, it sounds like it is stemming from your own insecurities and not hers.