Why do all of my posts seem to start with "it depends..."
The most important check is the wire gauge for the inlet and branch circuits. The inlet wire must be at least 7 gauge and the branch circuit wires need to be a least 10 gauge. (see note at bottom) It is possible you will need 4 gauge wire on the inlet - more below. Remember that thicker wires have smaller numbers (strange but true.) Your friend may get lucky as some boat manufactures run internal wire for 110v systems on all their boats and then just install the appropriate hardware for 110v or 220v. If the wiring does not meet these specifications he has two choices:
1a. Sail the boat to Europe and enjoy life
1b. Run away, run away...
Replacing the internal wiring will be both very expensive or if he is a DYI kind of person a royal pain in the posterior.
2. Check the AC circuit breakers. Since you are dropping voltage they will be too small by half so they will need to be replaced.
3. Replace the 220v shore power input with a 110v shore power input. If the 220V input is for 16 amps that would be a 30 amp shore power input. If the 220v is a 32 amp inlet that would translate to a 50 amp inlet (and 4 gauge wire for the inlet. Maintain the polarity color coding - brown or black to black; blue to white; yellow-green to green.
4. He could replace the outlets on the branch circuits but an easier solution is to purchase plug converters from a travel store. He should be paying less then $5 for a plug that changes the pins from Europe to US.
5. He may
need to replace the battery charger. Newer battery chargers might
be "world chargers". These are chargers that accept in input voltage of about 90v to 250v and 50Htz to 60Htz. If that is what is installed he is home free. New battery chargers run from $100 up to about $600.
6. If the boat has a 220v gen set he has two options - a step down transformer or replacing the generator on the gen set. The second option is a better choice - I would contact the manufacturer for advice.
7. Now the most expensive parts:
7a.The inverter. If the boat has an inverter or inverter charger you are in for a replacement. Most inverters only accept a limited input voltage - 110v or 220v. And the inverter on the boat is going to output 220v. He is going to have to replace the inverter. Again costs vary but we are talking in the range of $500 to $2000 for a new inverter charger. My preference is Victron Energy (Dutch) but Magnum (Nederland) has a good reputation too. Remember - friends don't let friends by Xantrex.
7b. Air conditioning. Again - if the unit runs off AC - as most do - you are going to have to replace the compressor and fresh water pumps. In other words pretty much the entire unit(s). So figure $3,000 - $4,000 per unit.
7c. Dual voltage appliances: Some refrigerator units run on both AC and DC. You would have the choice of disconnecting the AC side completely or replacing the unit
8. You might also consider that part of the cost of conversion would be new appliances - TV. toaster, microwave, etc. I don't know what is on the boat so I can't comment.
For parts, time, trouble, etc. I would be willing to pay about $5,000
(without air conditioning and a gen set) to $15,000
(with air conditioning and a gen set) less
for this boat than one wired for 110vac.
I hope this helps.
Note at bottom:
It has been suggested that 10 gauge for the inlet and 12 or 14 gauge for the branch circuits is common and adequate. I took the wire sizes from a specification chart - if your boat is wired with 10 and 12/14 wire I would think it might
be adequate (note the weasel words so you can't sue me!)
Addendum at bottom:
I forgot to add that your hot water heater may very well also be an AC heater - they usually are although propane, diesel, and kerosene heaters exist.
Fair winds and following seas...