I thought I had IBS-D, but what I really had was wheat intolerance. I keep away from wheat and I don't have problems with metformin, like I did when I was eating wheat. If she also low carbs, then less wheat is eaten anyway.
My advice would be to start the metformin at a time when she is not going anywhere such as over a weekend and stay at home. The second day I took it, I went out shopping, I had to find a loo three times before abandoning my shopping trip.
Don't let this put her off, some people actually get constipation with it, others get no side effects at all. If she doesn't take it, she will never know if it would have affected her.
Metformin SR is kinder to the stomach, so if she has problems, go back to the GP and insist on this, they prescribe normal metformin because it is cheaper. There are other tablets that can be tried if she can't tolerate it at all.
Metformin at maximum dose only lowers blood sugar by 1 - 2 mmol anyway, most changes to blood sugar are dietary anyway.