I'm 98% percent positive that FIL will not expect us to bail him out. He will pay for it all I'm pretty sure and hubby and I have discussed this. We will not be paying any more than what we spend and we will give MIL the funds a few days after the event.
It's going to be interesting when the bill is handed out at the end of the night.
Thank you for your previous update -- The In-Laws had a huge hoopla on their 40th anniversary.
Now FIL wants to do another huge hoopla only five years later? Then I totally agree that the onus is on FIL and you shouldn't worry a whit about how it's paid for.
I still wouldn't talk to your MIL about this. If there's an argument between them after the fact? Then let them work it out by themselves. Their finances are their business.
Instead of giving MIL the funds a few days later, I think that you should include a check as a gift, with your card, at the time of the party. Just like you would at a wedding reception.
Sending a check later and saying something like "I know you couldn't really afford this, but here is our contribution to the party" is rude in my book.
Give them a monetary gift that you're comfortable with, along with a heart-felt card and congratulatory note, then turn your head at any other conversations concerning the cost of a party that someone else (your FIL) is hosting.