Find out what voltage is required. If you're using one of these (or similar)
https://cdn.manomano.fr/20-x-9v-duracell-plus-alkaline-batteries-15v-pp3-smoke-alarm-10-x-2pkbat109-L-998074-2879501_1.jpg
then it's 9 volts.
If, more likely, it's some of these
http://www.bestdealsupply.com/mm5/graphics/00000001/DAAB.jpeg
(or any other type of cylindrical battery) then it's 1.5V per battery. (So if you're using 2 cylindrical batteries it's 3 volts. If you're using 4 of them it's 6 volts. Rechargeable batteries actually deliver slightly lower voltages but that's not relevant here).
Determining the current required could be trickier but an illuminated wreath, using LED bulbs, might need 4 watts of power, so (with a 6V power supply) you'd need to be able to supply two thirds of an amp of current. (4 divided by 6). Something like this could handle that
https://www.maplin.co.uk/p/regulated-6v-1000ma-dc-power-supply-l66bq
(but they're available far more cheaply elsewhere. Maplin stuff is good but ruddy expensive!)
You'd need to get your electrician friend to work out the total current required and then to decide whether to use one very powerful power adapter (which could be very expensive) or several low-powered ones separately for each set of lights (which might actually work out a lot cheaper).
Then all he'd need is a way of connecting the cables together, such as using the inexpensive connector blocks which he probably keeps in his toolbox anyway.