Firstly, sorry to hear about that. It’s an absolute shitter having your means of earning a living nicked off you and knowing they’ll be fenced on for ****-all somewhere.
I weighed all this up recently myself, with a few tool purchases taking me beyond what could be sensibly stored in the under-stairs cupboard anymore. I now pay £70ish a month for secure storage in a self-store place near me, into which all my work related stuff goes. Van is nice and clean all the time now and I don’t wreck the hall etc dragging my shite in and out all the time. I purposely have no bulkhead in the van so that anyone who wants to have a look can see there’s nothing in there.
I never ever store power tools in my van. Just a bucket of bits and some hand kit and the hoover. I’m not saying anyone deserves to have his kit nicked, but keeping them in your van, you’re asking for it. You say he’s had them nicked twice in six months – and he’s considering doing the same again?
Depending on what he needs, self-storage for a unit big enough for an average chippies kit might work out quite cheaply for him – certainly cheaper than replacing a bunch of kit twice in a year. He can claim it back as a business expense on his tax return and pay a small amount of insurance if he needs to.