Bowel blockage - magnesium safe for cats?

Hi! So a few days ago we found a cat (well, he found us, wanted some food, and ran away two days later). For those two days while the cat was here, our other cat (that is incredibly afraid of everything) was so afraid of the new cat that she was all squeezed and wouldn't even go to the bathroom, nor eat much. Now she has problems pooping because I guess everything hardened in there. So, can we give our cat some magnesium to help her loosen the poop (we have both cloride and citrate at home if one is better than the other, thouh I assume the citrate is a no-go)? We're still waiting for the vet to come (for some other issue), but would like to help her in the meantime.

Hello!   We wouldn't recommend giving Magnesium in this case, but contacting your vet again to advise them of the change (incidentally, if a cat has trouble weeing it is desperately urgent to get help and stress from new feline housemates is often a strong contributory factor to urinary blockages, so it is worth being very sure to check that your cat can urinate despite the constipation).