We cannot and will not advise anyone not to follow the advice they've been given by their medical professional. The NHS advice is
Can I drink alcohol while taking metformin? Yes, you can drink alcohol while taking metformin, but it's best to drink no more than 2 units per day. Drinking more than this can increase the risk of low blood sugar. Try to keep to the recommended guidelines of no more than 14 units of alcohol a week.
The risk is low blood sugar. Metfomin lowers your blood glucose, and alcohol tends to do that as well, by suppressing your liver's ability to add glucose to your bloodstream.
I have never been on metformin so have no relevant experience. However my tolerance for alcohol these days is much less than it used to be. I was also advised by a dietitian that if I was going to drink alcohol in any quantity, I should have enough carbs at the same time to ensure my BG did not drop too low. My choice is to keep my carb consumption down, thanks.