Stomped repeatedly by your training partner/peers, story of my life.
In my opinion, there really isn't anything you can do, especially with certain people and certain mindsets. It's definitely a frustrating experience for both people. You get testimonies like, "It's so frustrating, it's mind numbing playing you, I just can't think against you" and "I don't know how to even open against you, but you open on me 50 million different ways." If you sandbag, you hear "Wow you can beat me when you sand bag, cool!" or "I can't even beat your alts" and if you don't sand bag you hear, "You're not even trying, come on" or "Your alts don't matter, it's not like you use them". If you give them advice, they say "You can just beat it with X, like you always do" or "I don't see it/I can't do it yet"
The game becomes this constant push-or-pull/tug-of-war to get them to play with you, or to help them remain/become a happy/happier person, and it's kind of impossible.
Truth be told, it's up to your training partner's mindset to stay positive. The training partner has to stand up and stop cycling into a defeatist "You can just beat me with X" attitude, and reflect more on what they can do in different situations. It's on the training partner to go into the game with an open mind and change the way they see or don't see openings. The training partner has to say "Okay, against X I can Y, Z, or W in certain situations. I don't think Y is that good, I'm good at W but it only works against this rare mixup, and I'm bad with Z so maybe I should practice Z" or "I have Y, Z and W options, but I don't like that. Maybe I should switch characters because I like their options."
That being said, there are some encouraging things to do, as friends - and note, some, if not all of these, are personal testimonies from the bodied training partner:
-Get them to go to tournaments, and when they're playing, cheer for them. "OHHHHHHHH" for every big hit. Yell out only positive things, like "Lets go X!". Only say things like "Don't worry about it" if, you know, they actually won't worry about it. If he/she wins, be genuinely happy. If he/she loses, don't get down on their mistakes, let them talk to YOU about what HE/SHE felt about the match and THEN give them advice.
-Encourage them to play and go to smashfests. "C'mon, get your controller and play me!" It's really nice to have a lot of people at a smashfest or even at a tourney come up to you and say, "Yo what's up lets play some friendlies!" If you see someone in your pool who you think is on their level, tell them to play your training partner. They'll even establish a closer bond to that person/region
-Talk to them about cool smash things outside of their game. Spark interest in the game. "Did you see Ken win the crew battle at Kings of Cali for SoCal? You gotta see it!" "This commentary by Papi DAV3 is hilarious!"
-If he/she is in a good enough mood, talk to them about their game. "That combo was cool" "How'd you do against x player? Did you get destroyed? You took a game off of him? That's really good!" "What are your thoughts about x matchup? You think it's 50-50?"
-This one really depends on the scenario of who your training partner is: Play dittos against him, or play someone else (on a rotation) with his/her character. Tell him/her the point of this is that if you see something you like - a combo, an option, movement - your training partner can take it and add it to his/her own repertoire.
In doing these things, hopefully your partner can realize that no matter how crappy they think they are at this game, or how stupid/pointless it is to play, at the very least, you established a good relationship with someone over the game and that having nice friends is a real cool thing to have. And that because of it and their own interest to the game, continuing to play and try to improve may still yet be worth it, to friends and themselves.