I'm predicting he's going to crush you. So hopefully that serves as some motivation. ;)

Seriously though...I think you may be underestimating his bike. Neither of you have power meters, so I'm not sure if your virtual power FTP from TR is inflated, or how you know his FTP is 177. And while you indicate he only rides 25 miles a week, he's got a couple of 112 mile and 56 mile rides in him just from IM and HIMs. I have to assume he has ridden more than 25 miles per week in the past.

What are some of your TT results with your estimated FTP of 255 and what are some of his bike split results with that estimate of 177?

If you're doing 40k TTs in 1:05ish and he's doing Oly bike legs in 1:25...then yeah, you'll probably gap him pretty bad on the bike...but I stand by my prediction that he'll crush you just so you have motivation.

I think you need to bike easy enough to avoid cramping or extended periods of walking on the run. If you already cramped during an open 5k, then that means you need to bike pretty darn conservative if you at least want to shuffle the entire 10k run.

I see you don't list any multi-sport experience but he has some.
Have you ever run immediately after a hard cycle?

If not, do a Brick session today or tomorrow!
Some people stuggle A LOT when they first try running off the bike. Your legs will probably not feel quite right for the first few minutes of the run. You'll also likely be inclined to run too fast because you've become used to the sensation of travelling quickly on the bike and suddenly you're going a third of that speed. So practice a transition going from the bike to a sustainable run pace. Even if you only have time to do it once.
I know a guy who was/is a much better runner than me and first time he did a duathlon he basically shuffled the entire second run because he just couldn't deal with the switch from bike to run. He swore never to do a multi-sport race again and I don't think he has! I think that's an extreme example but still - if you want to beat him, do a practice!

If you're prone to cramping you may find that switching from bike to run could trigger calf cramps. Hopefully not.
Since it's your first go at it, I'd recommend backing off a little for the last minute or two on the bike and increasing cadence. That should help loosen up your legs a little. You can also stretch your calves a little towards the end of the bike leg without losing significant time. Just freewheel for a few moments and push your heel down (assuming you use clipless pedals). It could be worth a lot of time on the run if it helps keep your legs comfortable.