"Is there any difference in meaning between 給湯器 and 湯沸器?"
Technically speaking, yes, there is a big difference.
A 湯沸器 is a little water heater generally installed over the kitchen sink. It has its own tap from which you get hot water.
A 給湯器 is much larger than a 湯沸器 and it can be located anywhere in the house or apartment. It can even be located outside the house. It produces hot water for the entire house/apartment --- the kitchen, bath/shower, anywhere where tap water runs.
In other words, if you have a 給湯器, you do not need a 湯沸器.
" is there any difference in who uses 給湯器 and 湯沸器?"
Careful speakers/writers would not use the two words interchangeably. Some people call a 給湯器 "ボイラー".
Naturally, I have no idea what kinds of housing your coworkers grew up in, but the 湯沸器 has been less common by the year for the last 20-30 years. I myself have never lived in a place that had a 湯沸器 in the last 25 years or so because all of the places (4 of them all together) I have lived in that time have had a large central 給湯器 in them.
What this could mean is that if someone is in his 20s, there is a big possibility that he might never have grown up with a 湯沸器 around him. If so, the word 湯沸器 may not be in his active vocabulary.
When that person is old enough to leave his home and live by himself in an inexpensive little apartment that has a 湯沸器 but not a 給湯器, he might well call it 給湯器 because it still has the same function though on a smaller scale --- make hot water.