1 half loses interest in , is infidelity justified if kept discrete?

A couple married for 18 years find one half of the partnership completely loses interest in the other does'nt,. After 2 years things are clearly not going to improve the partner retaining their libido is otherwise a devoted partner and only misses physical intimacy, consideration of no strings with another begins to seep into the mind but it worrys said partner. Advice from either side please.