My fiancé has a drunkard, terrible ex. She sent me a screen shot of her phone saying he sent her a text but it’s not on his phone. He said he didn’t send it. The number at the top appears to be his in the screen shot. It is in the same texting flow as a conversation that had happened earlier. Is this possible? The screen shot shows it is part of the conversation. Not a standalone text.
To echo James' response. It is entirely possible for someone to send you something for a number that doesn't belong to them. This will usually be criminals trying to get you to click a link and steal credentials. Whether this is likely will be dependant on the nature of the message received. If the message is generic and contains a link then sure it's a possibility.
If on the other hand the message is of a personal nature then the above is highly unlikely. It is far more likely to be a message that the originator is attempting to lie about sending. It takes 2 seconds to delete a single text message from a comment chain on a phone and requires zero skill. Without further information on the content of the message this is by far the most likely scenario.
There is a third possibility that someone in the friendship group sent the malicious message but I wouldn't even beginning digging down that path unless there is an specific individual with a grudge that has advanced technical skills.
Finally, the screenshot itself could be faked. There are plenty of web-services that allow you to produce fake text message chains.
A screenshot doesn't prove anything. At all. As long as you know how to use Paint you can modify anything you want.
So the screenshot could be fake.
Or he could indeed have sent that message and then deleted it.
Conclusion: you can't know based on just the information you currently have. You'll have to find more information:
If it was an actual text message (not an iMessage, WhatsApp, or whatnot...) and your fiancé's itemised billing includes the dates+times of all messages, then you could see if a text was sent at that time.
You could ask to see the ex's phone yourself. Though there are probably ways to fake the message anyway, depending on the type of message/app and the ex's savviness.
Or, you could just trust your fiancé and ignore all that.