As others mentioned your "base" won't be the same after you added flavorings nor will be the nicotine. Then again, most store bought isn't accurate either when it comes to the amount of nicotine, but that's a different discussion, therefore that might be not as bad for you. However there's other factors that play a role here, and not going to work with your friends example, and that is the flavor manufacturer.
What this means is, while the majority of flavoring is PG based, there's also VG based and others. This aspect plays a magnificent role on the outcome of your juice. Other factors are depending on manufacturer the percentages will vary a lot, altering the juice and enjoyment in general. For example I could throw 20% flavoring bought from CAP in a juice, going by your friends example and probably would get something somewhat enjoyable juice. If I do the same with FA or TFA/TPA I'll end up with either very muted juice, or unvapeable or even one sided flavorings, since most of them would need to be either layered, used at lower percentage, or overall less flavoring.
Another thing might be worth mentioning, what kind of cotton candy we are talking about? Unless you've done your research let me tell you, not everything labeled as cotton candy does taste or works as such. Most manufacturer sell EM as cotton candy, and while that's basically is cotton candy, you would only use it to sweeten the juice, or mute harsher flavors, which by itself would make more sense to buy a more specific "flavor" as in magic mask or things like fresh cream, meringue etc.
If you actually wanted cotton candy flavor and feel, most likely you would have to buy some flavor that's called anything but cotton candy, or pick the two or three manufacturers that actually do just cotton candy. Most likely you end up with marshmallow, yeah I know not cotton candy but again, trying to show that without testing you can't follow a label or name lol
I know I'm getting a bit off topic here, but I felt the need to go a bit more into detail, diy ain't easy and needs lots of practice and patients, as well as time and willingness to do research. So to answer that question, yes you can throw it in as your friend advised, but most likely end up with what we call " tastes like ass" juice.
If you are more looking for actual enjoyable juice, you then can not go by that recommendation, and should as others recommended use a ejuice calculator and proper nic, VG and PG as well as flavorings. You can still use a base, but should get a higher nic one, if you plan on adding these flavorings etc, still would have to dilute/stretch with VG or pg at the end tho. If you don't want to learn that, or don't have time, stick with vendor bought juices and call it a day.