IR35

An acquaintance having reached 65, was retired by his large plc employer a few years ago.  Unfortunately, they swiftly discovered that his specialized engineering skills were something they could not do without, and so he formed a limited company and now toddles in fairly regularly to continue the projects he was involved in before retirement. (The work is so specialized that whilst there is a substitution clause, it is unlikely it could be used)

From an IR35 perspective, how much of a risk is he at?  He had no choice but to form a company, he had no choice but to retire.  He has paid his full 40 odd years of NI and draws a state pension + SERPS.  (It strikes me that it would be immoral and unfair for HMRC to have a go at him simply because he might fit the usual IR35 target profile.)

Anyone else dealing with the elderly IR35 potential victim?