If you were previously underweight and you gained 10lbs, odds are, you are exactly where you need to be.
Before I had my daughter, I was 5'2'' and 130lbs of muscle. A friend of mine was flipping out because she was 5'10'' and 130lbs, and she viewed that as fat. She was super tall and very skinny, and I was short and proportioned.
I was in a size 7-9 jeans before I had my daughter. 3 years after I had my daughter, I'm in a size 4-5 jean. I just don't have the muscle I used to. I got that muscle riding everyday, training foxhunters and polo ponies. Now I just ride one horse, maybe twice a week if I'm extremely lucky.
Its good that you gained weight. Though, your friend might not understand the muscling it takes if you take her riding. She will be restricted to just the walk since she doesn't know much, and making her do anything more to prove a point would be kind of dangerous.
Can you take her on a trail ride? That will make her use those muscles while walking the whole time, and she'll be super sore afterwords.