The switch to a coolant mix will, in itself, not cause any problems. But the previous running on water, without additive, may already have caused problems that only manifest themselves now, e.g. fouling or blocking of coolant channels with rust. This can lead to hot-spots, gasket failure, etc.
Depending on how badly and how long the system was neglected, a flush might not be able to remove all rust residues. I was in this situation. The previous owner of my Fox had only filled up water. By the time I got the car and did a flush, it was too late. A few months later, the head gasket failed because of a rust blockage that had remained behind even after flushing, causing a local hot-spot.
So, flush the system and use the correct coolant mix, but continue to observe and be prepared for underlying problems. If the engine doesn't lose coolant or overheat, your friend was lucky.