Hey Calanga and welcome to the forum.

There isn't anything risky about this.

The only extra thing the other Manager can do if you're his friend is organize a friendly match - When you're invited to a friendly match you will be the visitor. Only the Home team will be affected. So only players from home team will improve, lose morale and condition, and may be even injured.

To be honest some Managers don't seem to notice that such is the case and believe that if they have you on their friend list they will ruin your team's condition and sabotage you right before important matches against them or other opponents. Such is not the case though.