Ahhh you guys are right. I should be telling her in person. Reading back I supposed I am dodging the issue a bit, I just feel so awful saying "you can't ride my horse because you're so heavy she may injure herself." I suck at stuff like this. I feel like suggesting that I help her lose weight implies that I don't think she's trying hard enough. I only know what she tells me, but she does seem to be trying, she runs every day, she says she eats healthy.
I know no one is going to sympathize with me on this one, but what makes it so much worse is that I don't even kind of have a weight issue, I'm honestly in really good shape. I'm 5'6" and 130 lbs. My diet consists of vegetables, lean protein, fruit and whole grains, and I work horses all day long. So, I don't know if anyone understands that, but it sucks telling someone they have that issue when I can't even say something like "yeah, I could lose a few pounds too, lets work on it together" no, it's not about me at all, I just feel like its even more awkward because of that.