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The SOHC V6 motor in these is notorious for having issues with the timing chain components, and the guide likely did need to be replaced if that's what you have. The OHV V6 motor doesn't usually develop timing problems and replacing the guide would be a very minor thing.
It sounds bizarre that a shop would blow a head, or even a head gasket, from working on timing components. That makes it sound like this shop doesn't have a clue what they are doing and your problems have all been caused by whatever else they did while messing with the vehicle. You might try getting the shop to either fix the problems they caused, or have them pay for a competent shop or a Ford dealer to do the work and make it right.
It may just be that they didn't torque the exhaust manifolds that are attached to the heads, or perhaps they didn't bother to use new exhaust manifold gaskets and so the old one is leaking, or they might not have used a gasket at all. Exhaust gaskets are inexpensive and fairly easy to replace with enough tools and time, just removing a few bolts. It likely won't be a cheap fix when paying a shop to do it, but it still shouldn't be anything like a timing repair. Again, I think the shop you went to should fix this for free, or pay to have it done somewhere else, whether it be just re-torquing the manifold bolts to seal the leak, or installing a new gasket. Hopefully the other side has a gasket on as well, otherwise they should install new gaskets on both sides.
If there is a lean running code caused by an O2 sensor, usually the oxygen sensor on the side that is running lean is the one that's reporting the code. I would guess that if it was a sensor, it would be the sensor closest to the engine on the right bank side if that's what side the code is reported to be from.
You need to have that exhaust leak fixed first though, since an exhaust leak can cause the sensor to think the engine is running lean on that side, because some of the exhaust is going out the leak before it gets to the sensor, and the other side without a leak is, by comparison, normal. Repairing the leak may eliminate the code and no sensor replacement may be required.
