Mutually agreed boundaries can be helpful and a relief to the person struggling. However, I'd also note that if there is resistance to or pushback on boundaries around a past bad behavior, that's likely a bad sign all around. Yes, people can change - it is hard, takes time, and often comes with setbacks. If someone has not honored your boundaries in the past, they are not likely to honor them in the future (or respect new ones) - it's not impossible, but it's also not likely.