A couple comments.
First does she bike a lot, if so skip this comment. Poor form while biking or an ill fitting bike can cause a lot of knee pain.
Second assuming that you are walking 20KM a day, a good amount for a walker, that is a 2-3 max hour bike ride unless she is planing a side trip for each day. Some of the places people stop for the night are not full of churches to see or other things to do for the 3-6 hours waiting for the walkers to reach the stoping point.
Third the route is very busy at that point and not about being polite but that the route is not very wide at some points vs the number of walkers. However there is a well maps out road route near main route.
Fourth she/they, who ride bikes will not receive a
compostela if starting in Sarria that matters to them.
Fifth she/they can rent an e-bike which will help with the hills to cause the ride to be a little easier.
IF the plan/point of this is to spend time as a group but you need two set of people one on bike and one walking. I would have the walkers start in Sarria as plan and the biker start in
Astorga at 250km or
Ponferrada at 197km with a ride back to the 200km stamp point. each group planing to meet up for the walkers last day into to Santiago and spending time together in Santiago. Maybe if there was time have the whole group spent a day in Astorga together before splitting up with the bikers riding out and the walkers taking the bus to Sarria.
Please note that the route from Sarria does include hills.