I assume you mean the person feels insecure. Does the person feel insecure because:
(a) he/she is paranoid/pessimistic/lacks self confidence, or
(b) his/her partner is a cheating, lying scumbag?
If it’s (a), he/she should try to gain self confidence. I don’t know how one does that. Work on physical appearance: lose weight, gain weight, start excercising, get laser eye surgery, braces, a nose job, a boob job or whatever. Actually, excercise is good mentally too, so that’s a biggie. Learn a skill, take up a sport, turn off the TV, get off the fucking couch, quit drinking/smoking/etc. But poor self confidence often results from years of emotional abuse from family, classmates, partners, etc, and it may be impossible to change that or at least would take years of therapy and/or a change in partners/friends/family members or other circumstances.
If it’s (b), dump the loser and try to find someone else who’s not a loser. Or maybe don’t find someone else. Maybe he/she would be better off dumping the loser and spending time working on his/her self confidence.