I can't use the pins to power it since I'll be using both the 5V
Are you using a display on a shield, or wiring your own connector? You can certainly wire your own 5V input in parallel.
since it needs 5V 2A to run and the batteries' voltage and amperage differs.
OK, you or your friend are suffering from a very common misconception about power supplies (and two-terminal components in general). A power supply (whether it's a battery or a wall wart) cannot dictate the voltage and current through a load simultaneously. Just one, or the other, or some current as a function of voltage.
See this thread, and where it talks about a power supply, think "my battery".
Two Lithium-whatever cells in series is going to be too much voltage for a Raspberry Pi. So you need something that's regulated at 5V, and that has the capacity to drive enough current (note the difference between "regulated at" and "has the capacity").
I'm assuming you want battery power. If not, just get a USB charger (or a Pi kit that includes one). If you want battery power, and you have the money, the easiest way to arrive at that is to get one of those USB charger packs, and make sure it has enough rated current to power your Pi, your display, and whatever else you're going to stick on there. Then just plug it in and go.
The next easiest thing to do is to use your 18650 cells and power the Pi through a switching converter (since you're going down in voltage, you want a "buck converter").
I'd like to say that you want to get one rated at 5V and 3A -- unfortunately, there are a lot of really enticing-looking, really badly built switching converters on the market, that don't deliver power to their ratings. If you buy stuff off of mass-market sites, then go way over on the ratings. If you buy name-brand devices from reputable distributors (in the USA: DigiKey, Mouser, Newark, Avnet), then you can worry less.
