If she's flying JFK-LHR-FCO-LHR-JFK then she'll be arriving first of all in to T4 not T1. The JFK flights arrive/depart T4 and FCO arrive/depart T1. 2hrs is not a lot of connection time really to make it through everything. My advice would be:
1) Arrive T4. Follow the Flight Connections Centre signs to T1 and stay airside (at JFK they should be able to give you the LHR-FCO BP if it's all on one booking). If she's in Club Europe as you say, she'll be able to use the Fast Track at the FCC too. Once in T1, head for the Terraces.
2) At T1, see what time she has left, but if all goes well she'll be able to have a refreshing shower at the Terraces (but not use the MB spa as it's for F/J/Gold on longhauls only).
3) On the way back, do this in reverse. Arrive at T1, use the FCC (Fast Track again if she's onward to the USA in J) and head to T4, not bothering to go landside. Once she's at T4, she will be entitled to use the T4 MB Spa since she's in longhaul J...
but with only a 2hr connection I doubt she'll be able to get an appointment, sorry
Again, however, a shower at the Terraces will be an option (go to the ones by Gate 10, not Gate 1...the MB Spa is also there if she wants to chance her luck).
Staying airside using the FCC will save a lot of hassle, and the showers/snacks/drinks will be OK in the Terraces in both terminals. 2hrs just isn't enough of a time to get from terminal to terminal and use the spa in either direction - so all she'd really miss out on at T4 Arrivals is the breakfast buffet.
Phil