I have a customer who has 25kW of PV on site and would like to heat his hot water cylinder and switch a fridge freezer , using energy that would otherwise go to the grid. I could install an iBoost or immersun and a separate Divice to switch the socket, however does anyone know of a device that could do. More importantly has a recommendation .
NB; the array is not his he has free use of the energy, it is a three phase installation and the freezer is used for freezer packs not food.

Some inverters (SMA) have a built in relay that can be used to divert power. So check out the inverters first.

Our old mate JSHarris built his own diverter, you could hunt his design down.
https://openenergymonitor.org/forum-archive/node/2429.html

Possibly? you may have missed the Solar and Storage live exhibition at the NEC that ran 3-5 October. It may still be possible to get an exhibitors catalogue to steer you through some of the many manufacturers of power diverters etc.

I looked into it some time ago for my array, but you may find there are some obstacles e.g. 3 Phase, large array and the ability of domestic diverters to cope, Power company /DNO certified, not all are. I seem to recall Immersun was one of the few that they recognise. That may have all changed though.

Is there a problem with two separate devices? Many diverters will handle the heat the hot water cylinder requirement and I'm sure I've heard of at least one device that will switch a device that requires at least a specific power availability (sorry, not sure what but I think it was on here).

Does the customer have access to the inverter's parameters?

I'd have thought that if there is a customer involved then the various product and safety laws would disallow a homebrew solution

For a commercial offering, the iBoost seems to hold the best cards.
Regarding DIY: the JSHarris, Wooldridge, and my 2012 homebrew project for that matter all make use of a commercial power controller, which is an expensive part and makes the setup not very efficient at low power levels.
Robin Emley's solution is a) more efficient, and b) can do multiple devices of varying power ratings.
http://www.mk2pvrouter.co.uk/33001.html
For more than 2 devices the hardware needs to be expanded and additional triac switches built. It is not too difficult though, and the triac switches are relatively cheap.
3 phase is more of a challenge, I think the folks at openenergymonitor have tackled that one a while ago.
Sounds like a nice project, and 25kWp generates a lot of power to play with. Good luck!