Advise a friend who manage conflict by conflict avoidance and explain why it is not healthy for maintaining positive relationships

Conflict avoidance is not the best strategy to cope with conflicts: if one is avoiding conflict, the conflict will not be resolved and for example people will not coordinate on their action or someone will do something that will hurt another.

See for example a conflict of the colour of the walls in the house: avoiding conflict will only mean that either the walls will never be painted or that one person will be very unhappy.

A better solution is a compromise.