EDIT: Cloudflare is blocking this post if I quote NEC code sections in numeric form so I apologize for the below reading a bit weird, it would not otherwise accept my post...
I am kind of surprised at how often on this forum we beat up this same exact topic over and over again. Electricians are bound by the National Electric Code. It has very specific requirements for safety reasons. Generally the code is somewhat conservative and so I would not have any issue using any code compliant installation. Going a little above and beyond is fine, but definitely not required. The real reason to go above and beyond would be because you intend to make changes to your installation later that may have other requirements.
So first off:
The source of truth for wire sizing (in the 2017 version of the code) is table "three hundred ten period fifteen B 16". You can register for free access to the National Electric Code from the NFPA web site to read it for yourself.
So that you don't have to register here is a handy copy of it with a bunch of the relevant code all in the same spot:
http://www.barr-thorp.com/wp-content/uploads/2011/04/digest-176-nec-tables.pdf
You can see that 8 gauge copper wire is rated for 50 amps if you are allowed to use the 75c insulation ratings. If you are using NM wire (Romex) it must be run inside the wall and there is a limit that says you must only use the 60c sizing column. So #8 AWG is NOT ok on a 50 amp circuit if using Romex. If the wire is run in conduit or is metal clad wiring then you can use it because you can use the 75c rating.
To the distance comments- I have not read the NEC voltage drop sections in depth, but from what I understand, the voltage drop sections are guidance and not actually rules. It is good practice to follow, but not a safety hazard. More of an efficiency issue and if voltage was low enough it could make some appliances run worse or not have as long a life span.
To others comments: Yes, you could even use 8awg romex (NM wire) on a 40 amp breaker using the NEMA 14-50 receptacle and be totally fine and code compliant with your Gen 2 UMC, but I would not recommend this since a UMC gen 1 would be a problem (since it takes 40 amps) or some future other charger might be the same (note that EV's are considered continuous loads and so you must size the wiring and breaker 25% larger than their rated ampacity). The Gen 1 UMC draws the full 40 amps and so it must be on a 50 amp rated circuit.
But with all of that being said, I would probably run #6 anyway (that is what I did) since it is probably a somewhat minor cost delta and it gives you a little more flexibility. Your car will be one of the largest (if not the largest) power consuming draw in the house and so a little more efficiency (less line loss) would not suck. Others have pointed out that with #6 wire (if in conduit) can be used on a 60amp circuit to feed a HPWC which will then give you 48 amps usable for the Model 3 which is the max the Model 3 can make use of. I also did this for my wall connector.
To the comment of using #3 wire: That would be a large cost delta since you would need larger conduit and wire plus it is a royal pain to work with. It would not fit under the terminals of the receptacle or probably the breaker, so I really would not recommend this route unless the specific placement of the receptacle is exactly where you would later want a wall connector *and* you were planning to install a wall connector later. Copper is super spendy right now!
So yes, your electrician is correct as long as he was planning to use wire in conduit and not just Romex (NM) cable.
