Heidi,
This is such a fun idea!

We've actually had many families do LHTH in all sorts of settings, even though it was written with the home setting in mind.
Depending on how much time you will have when you meet once each week, you could either do one entire day's worth of plans or two day's worth of plans. I wouldn't do much more than that or you'll lose your kiddos' attention.
In order to get the full benefit of LHTH, you won't want to randomely be skipping activities in order to squeeze more in during your sessions. This is because everything within HOD has a specific purpose and skill attached to it, so in order to be ready for LHFHG, you'll want to do LHTH as close to the way its written as possible.

Otherwise the program will lose its flow, its sequential building of skills, and the thread of God's continuous story through His Word.
If you will be meeting with your friend each week, you have two routes you could go with LHTH. One would be to not worry about tying your own LHTH schedule to the weekly meeting time but rather just progress with LHTH on your own at home and make the time you meet be a review. Little ones love to repeat activities, and it can sometimes be hard to tie yourself to someone else's schedule. In this way you won't end up hodge-podging your way through LHTH but rather will consistently move forward without skipping or feeling rushed.
Another option would be to designate which day or two of the unit you will be doing each week during your meeting, and then make sure that you have done the previous days in LHTH prior to your meeting time so that you are on the right unit and day to match with your meeting time.

If you do go this route, I'd plan on doing LHTH 3 or 4 days a week to allow yourself plenty of time to get in the days needed to be on track prior to meeting with your friend.

This would also keep you steadily progressing forward and would keep things in order sequentially.
Anyway, just a few thoughts as you ponder a plan.
Blessings,
Carrie