You are three weeks away from celebrating your coworker's last day with a proper steakhouse dinner. Getting that private room on a weekday night means locking down the headcount and the reservation early so you are not scrambling at the end.
A typical US night for a group runs about $850 once you factor in the cuts, sides, and drinks. Spreading that cost out keeps things manageable.
That 0% intro window is a deadline, not free money. If you still have a balance left when the 15 months are up, interest kicks in on the whole original amount.