Yes but at that point he'd already done his job as a fixer, that's why she made us the offer to betray him. She hired him remember so she can replace him. As for Dex, he was already suspicious of her, he asks you frankly if there's anything he should know after you meet with her. Betrayals are not unusual in a fixers world or clients. We meet a fixer in the game who was double crossed. Panam was also double crossed by her partner who attempted to sell the goods. Night City is rife with betrayal so there's nothing unusual about it. If Dex killed her then he could easily sell it to someone else, reputation doesn't matter because he already has a shady one. No one we meet in the game has anything good to say about him, I dare say having a bad reputation is his preference. It's why he went into hiding for years and only just resurfaced. It's why he took the job no one else would. Besides if Evelyn told anyone about the betrayal she would be admitting her involvement in it and likely killed. Dex's purpose was putting together a team and the team was already gathered at this point. T-bug made it clear through conversations that she has allegiance to no one. Evelyn also could find another fixer if needs be.This wouldn't make much sense since Evelyn is the client, and Dex is the fixer. Dex doesn't have any interest in the Relic, and is only organising the job because Evelyn hired him to do it. If Dex, as a fixer, decides that Evelyn is a bad client trying to cut him out of the deal, then he just blacklists her and no other fixer will take her jobs. There wouldn't be any killing here, because not only would he cut out the client if they decided to go over the fixer, but also anyone who took the job, which would be very bad for business for V. This is precisely what makes fixers so valuable and important within the Cyberpunk universe, but it's a two-edged sword. A fixer who killed their clients would also find business drying up really fast unless the client posed an actual immediate threat to the life and/or reputation of the fixer in question.
Ultimately, though, there's no job without Evelyn OR Dex, which is why V never actually cuts either out. If there was going to be any cutting, it would happen after the job was successfully completed, and V had a biochip ready to hand off to one or the other in exchange for payment. Since we never actually get that far, that choice at that stage becomes a non-starter.
V doesn't cut either one out because the writers opted not to. It very well could be written differently. It also doesn't need to progress after the heist. Things could still progress as they did and you take the chip to Evelyn instead of Dex. We don't spend nearly enough time with either of them for it to involve the story beyond that. It would simply add a layer for replayability.
The one thing the game struggled with is creating unique paths, more so with the life paths but there are a few missed opportunities like this one.