After noting Andrew's post, I will second his recommendation to consider other (and all!) possible sources of this friction. This should be a simple diagnosis!

Lift the chain off the crank and check that the cranks turn with little effort, then lift the chain clear of each derailer pulley, one at a time, and check for free rotation of each pulley.

Do note that having the bike in different gears, both front and rear, can really affect the amount of rotational drag felt at the cranks!

Of course, removing the wheel and spinning the freewheel is also a critical diagnostic imo, and might verify if perhaps the bearing adjustment is perhaps somehow too tight.
The freewheel might also be rubbing on the spoke protector if a spacer was omitted or if the protector became distorted or if grass or string, etc., has gotten into the space behind the freewheel.
Sometimes a freewheel will rub against a derailer hanger bolt when the axle position is changed even slightly during installation.

There are lubes that thicken as they dry (such as aerosol greases and motorcycle chain lubes), and possibly these may have been fed into the freewheel.

Grease in a freewheel has definite advantages, but only if strategically placed during assembly. Many freewheels (especially cheaper ones, or those that for whatever reason have more than an average amount of freeplay), will creak under power if the bearings aren't kept lubed, so of course greased bearings would extend the lubrication interval greatly, while the grease gets pushed aside just enough to allow totally free rotation. Too much grease (or a webby grease) otoh, can create a lot of churning for some time, and can even cause the chain to sag onto the chainstay while coasting as the reverse torque overpowers the derailer's tensioning spring(s).

Even if the mechanic's work is not suspect (and while the wheel is removed) I would of course also verify that some freeplay exists at the rear axle before the QR lever is clamped down.
If the axle is solid/nutted, otoh, there should be no freeplay.