I use 25L barrels with a PF trolley for all of my work. Occasionally I leave trolley in the vehicle and just run a hose straight out of the back like a small van mount. I've never had the need to connect more than one barrel at a time but I would imagine there could be an issue with airlocks if they were all connected together and one barrel ran dry before the others.
I don't know what his reasons for wanting to connect them together are but I think I'd be inclined to suggest to him to use a length of minibore as his pickup hose with a strainer on the end like PF supply for their trolleys and make it long enough to reach the furthest barrel if he doesn't want to lug them around the back of his van. I think he would probably find this less problematic than connecting several barrels together.
However, there is I believe, at least one window cleaner who has posted on one of the forums that he worked with barrels connected together for a while, though I think he now has a tank fitted in his van. Perhaps he'll see this thread and comment on his experience.