TL;DR: McDonald's still comes out ahead (soda is that cheap)

Jump down for the summary conclusion.

The cost of the soda: $35 cost price per 5 gal of syrup (McD wholesale¹⁾ with a 1:5 ratio of syrup to carbonated water. (By the way, I can tell this is McDonald's by the soda fountain.)

1 gal = 128 ounces
5 gal syrup + 25 gal water = 30 gal soda = 3840 oz
$35 / 3840 oz = 0.9115 cents per oz or 13.7 cents for 15 oz
$35 / 30 gal = $1.17 per gal

Average cost of:

large soda = $2.00
medium soda $1.50
value priced small soda = $1

As long as he didn't buy the small soda, McDonald's still comes out ahead on serving 1 gallon if he paid for 1 single soda. Even if he did buy the small, they lost a less than 20 cents on the transaction which could have been made up by buying 1 burger.

Adding in the CO2 cost. C02 is cheap

$15 cost price for 20 lb of CO2 which makes 275 gallons of carbonated tap water
$15 / 275 gal  = 5.5 cents per gal

Also not including the cost of the cup because he is using his own cup, but let's include it anyway:

$50 cost price for 1000 cups (McD wholesale^1)
$50 / 1000 = 5.5 cents per cup

$30 cost price for 1000 lids
$30 / 1000 = 3 cents per cup

Cost price: $1.17 soda + $0.055 CO2 + $0.055 cup + $0.03 lid = $1.31

Worst case scenario is he bought a small soda:

• Net loss: 31 cents ($1 - $1.31)

Best case scenario is be bought a large soda:

• Net profit: 69 cents ($2 - $1.31)

 ¹ Regular restaurant costs will be slightly higher because McDonald's has a highly efficient supply chain with enormous throughput of product.