I have an Opus BT-C3100 that I use for all my charging needs, but I'm buying a USB chargeable flashlight for a friend. She is not into recharging Lithium batteries and wouldn't know what to do with a 18650. How good are the USB charging mechanisms of flashlights? Is it safe to just use that for the life of the 18650 cells contained within the flashlight, day in day out? I imagine she would use her USB iPad charger plug unless I need to get her something better.
Short answer is yes, it is safe enough.
Keep in mind she is already charging Li-ions in her iPad (and Iphone i presume) for years.
I have refrained from gifting li-ion flashlight in the past due to battery considerations
but now days I have started gifting usb flashlights to people.
If you want to feel safer install a ‘safe chemistry’ cell the IMR type which also happens to be the best/most Amp cells anyway.
(30Q, vtc5,6 etc)
Also, do a charge yourself upon getting it to ensure the cell was not charged above 4.20-4.21 Volts.
I have used cheap chargers in the past that charged cells to 4.24-4.25 Volts,
this would raise some concern regarding cell life and safety.
It’s as safe as an external charger. There are good and bad external charger. The same is with the rechargeable electronics inside the flashlight.
I did buy the Samsung 30Q's based on a recommendation for the Sofirn SP30A I ordered (still coming). But like all 18650's, those warnings do make me worried. I can't believe the USB circuitry in a flashlight can be equal to a full charger. We'll see what happens.
https://www.imrbatteries.com/samsung-30q-18650-3000mah-15a-flat-top-battery/
But yes, I am doing a full charge-discharge-refresh on them with my Opus just to make sure before gifting the flashlight.
I always check charging current with one of the little USB Doctor meters when I get a new USB light, and check with a multimeter the end voltage of the battery when it is shown as charged.
@robertsig, the SP30A has been tested to have a good and safe USB charger.
You should not be worried then.
The battery is fine, not what you need to test.
Test the flashlight charging circuitry by doing a charge inside the flashlight, then check the voltage.
I bought the SP30A including a battery with a protective circuit as a kit. The Samsung 30Q high-current battery is unnecessary for the SP30A. The maximum flashlight current is about 3 amps. Better is a battery with higher capacity and lower maximum current. Panasonic NCR B, Samsung 35E, LG MJ1 …
I got the SP30A in and apparently made a few mistakes. It came with a Sofirn 2800mAh (protected?) battery already. It's working well so I have no reason to change. However, I now own a pair of pink Samsung 30Q's. Can anyone recommend a flashlight to buy so I can use them? 
What’s your budget and what do you want to do with it?
You can get the Nightwatch NS22, which comes with an SST-20 4000k. It’s a high CRI budget thrower, so you will probably like it:
That’s the spirit! 
SP32Av2. MH20/MH20GT. TK18. Go crazy…
Good choice BlueSword. On the list!...
BLF A6
Amutorch AX3
How do you know which lights are safe with protected vs unprotected cells?
Always check for low voltage protection(LVP).
If a light doesn’t have LVP (most Nitecore lights), then I would recommend only running it with protected cells, or like me, just not buy the light.
Get into the FW3A list, wait until it’s ready to ship and don’t buy any other flashlights in the meantime. Your wallet will thank you for that.
That might be the ticket ggf. Does it work with flat tops?