Oddly, I believe your friend has it right.
The dry-cleaning process is very similar to standard washing with water, except for the fact that instead of water, they use some sort of solvent blend designed to break-down various chemical structures and/or disperse them into the surrounding liquid, which is then extracted & filtered out of the solvent through filters and such. In a typical dry-cleaning, the solvent-extraction process is ~99% efficient or better. (meaning, 99% of the solvent is reclaimed for re-use) Which means that almost nothing is left behind on the garment when finished. What little remains is not going to be kept in the garments by a non-air-tight plastic baggie.
Those chemicals can also cause damage to the fabric and/or dyes over-time, which is why the dry-cleaning process tries to remove as much of the solvents as possible. As a general rule, the more aggressive the solvent, the better job it can do at actually 'cleaning' the clothing... but more aggressive cleaning can result in damage to the clothing. Even some of the plastic-bags themselves can cause the clothing to discolor over time.
According to "Arrow-Care",
... the plastic bags are meant only to protect your clothing while
transiting from the drycleaner to the home.
That simple plastic bag can help keep the clothing clean from various sources such as dust, microbes, accidental spills and various other sources... but beyond the obvious 'less-cleanings,' a plastic bag won't keep the clothes from deteriorating over time.
Much of the details can be found on wikipedia.