If he's not getting a reduced engine power light under hard acceleration then you can just live with westgate rattle. Mine rattled from 80,to 109,000 miles until they no longer held boost.
I replaced mine with a pair of genuine Mitsubishi turbo 's, £1,500 direct from Mitsubishi plus fitting.
Car was always quick, now it pulls like a train
No point going hybrid unless your going fbo.
Mines running 385bhp & that's plenty for me.
How many miles has it done?
If it's got westgate rattle it'll most likely also have some end float on the turbines, so not worth replacing just the actuators.
They could be rebuilt, I was quoted £450 each plus vat but decided to replace them instead.
I suspect the seller new the tubs were on there way out & sold it as they didn't want the expense of replacing them
If you want an idea of how much a 335i costs to run have a look at the how much has your 335 cost you this month thread