From a technical standpoint
A few months ago, a composer friend of mine got together with me to write a quick melody, and we did it by having him screen share the sheet music he was working on through Discord so I could look at it and offer suggestions. So I think you could definitely do this through screen sharing software, i.e. Zoom, Google Meets, or Discord. Have them pull up their music composition software and share the video and audio with you, so that you can see it and hear it - assuming it's something like MuseScore where you can play the piece in progress so the other person can hear it.
Alternatively, if you don't have access to that software, you can just have them send you the sheet music in progress and send them back a Google Doc or some notes about potential changes.
From an interpersonal critique standpoint
With things like music, art and writing, the person has put a lot of their heart and soul into the work, so all the usual critique guidelines apply here - be kind, phrase your criticisms as constructive, and try to do the "Oreo technique," where you say things you like in order to frame things you don't like as positive-negative-positive. For example:
"I really love how this stanza sounds. Maybe you can change that chord in the left hand? The rest of it sounds great though."
If they decide not to go with the critique, don't push it too hard unless you are adamant about it. They have agency in the process and can choose not to listen to you sometimes, and that's totally fine. You are both in it together, so you should have nothing to worry about even if you disagree on something - it won't ruin your friendship!
I hope you have fun on this project! Co-writing is a blast.