No real idea on disinfectants, but you could try Kemfast, who supply a lot of Chemicals to the aircraft industry (01292 479834)
As to setting up as a Spares Broker - no experience or qualifications needed at all - that's half the problem! A lot of individuals got into the business because all you need is a fax machine and a few contacts......my ex-boss made himself a millionaire with practically no knowledge of practical avaition matters at all - he could just rip off desperate customers. The more reliable and trustworthy vendors will join Trade Associations, but it's all voluntary. The CAA/FAA don't regulate them, the larger customers may audit some of the vendors, but there are still quite a lot of one-horse operations about, servicing the end of the market that doesn't want to pay top dollar.
If your friend has some ready cash available, spares are cheap as chips at the moment thanks to the market downturn - if he knows some good customers, get on their side, and look after them well...they will stay with him and keep him in business. Rip them off and they won't come back. Anyone can make money once, but building strong relationships takes time and patience. That's where the real success lies.