Since your friend has bored it out to .45-caliber, RUN to the nearest gunsmith to have it checked out.
Cartridges of the World, 8th edition, notes that bores on the 1873 French Ordnance Revolver are close to .451 inch across the grooves.
I agree with MikeG that this revolver probably should not be fired. While I don't know the thickness of the chambers in the French Ordnance Revolver, I agree with MikeG that many 19th century revolvers had very thin chambers.
And he's also right about metallurgy being suspect in some 19th Century firearms.
However, if you dooooooo decide to shoot it.
The original load was a 180 gr. bullet at about 700 fps.
That's a mighty puny bullet for .45 caliber, so don't exceed that bullet weight.
I should think that 185 grain wadcutters could be used. Cast them very soft. Lubricate the bullet with SPG, Lyman Black Powder Gold or even the classic beeswax/tallow mix.
Do NOT use Alox-based lubricants, or any other petroleum-based lubricant. Petroleum based lubricants create a tarry fouling when used with black powder.
Load this bullet into a case of 20 grs. of FFFG black powder. No more than 20 grs.
If the bullet doesn't slightly compress the powder charge, use an Ox-Yoke Wonder Wad of .44 caliber on top of the powder, to bring the level up.
You may also use a little corn meal on top of the powder before seating the bullet. However, if you use corn meal the bullet must be firm on the powder to keep the powder and corn meal from mixing.
Your first dozen shots should be done with the revolver secured to something, and the trigger pulled from a distance by a string.
Examine all cases for signs of excess pressure, and examine the gun for damage.
When firing by hand, wear eye protection and a heavy leather glove (to prevent powder burns if she lets go; the glove won't protect you much against metal fragments, however).
And don't allow anyone near you when firing it, especially to either side.
If it were me, I wouldn't fire that gun. I've never examined an 1873 French Ordnance revolver, so I don't know how stoutly it's made. Perhaps it's a strong design, perhaps not.
In any case, I'd have a gunsmith familiar with 19th century arms look at it.
An old, 19th century gun is not worth your eyes, fingers or nerves.