Based on the past two chapters, the other guy seems to be a host who she pays for intimacy. She gives him money in exchange for sex and spending time with her. He was annoyed that she stopped contacting him last chapter, so we can assume that she is one of his primary clients and his income depends on her calls.
The future drama will probably entail conflict between her false/transactional relationship with him and the chance of something genuine with the MC.
At the moment though, she seems quite inconsiderate towards the MCs feelings and functions almost like an emotional vampire, who drains him emotionally but cannot uplift him in the same way. MC is in for a brutal awakening from the carefully curated image he had of her as an idol. Though he knows she is deeply depressed, he still views their relationship from the persepctive of "I can fix her". Hopefully the author doesn't go with the pure NTR route and have him walking in on them fucking or anything like that, but MC will find out about it sooner or later.
At the moment them end up together would not be a good decision, they are both emotionally unstable, especially her. She needs to get her self together first, or she will just be an emotional burden that drags the MC down.