I'm a skeptic, but I found these interesting NASA resources after googling for information on the device:
I didn't look too much into this topic tbh, so I have no clue how much effect a device like this would have on other parts of the body. So I'm guessing it's also very important what the price is - and how "professional" (a.k.a expensive) it looks - for the end user. An individual that believes in these things will never accept that a $5 DIY kit will have the same "effect" as a $200 medical grade TenDlite®.
So unless you are able to make something look and feels as expensive as the "real" thing, I don't think it will convince a believer of these things.
Anyway, these are from the user manual of the TenDlite®...
1 Rechargeable Battery - Li-Ion 18650
300mAh3000mAh 3.7V Li-IonTECHNICAL SPECIFICATIONS
Model: 204 - Tendlite® Medical Device
Voltage: 4.5 Vdc at 1.0 watt input
2600 mW Input - 1500 mW Output
LED‘s Wavelength: Therapeutic Red 660nm
So unless you can find the exact same therapeutic red LED type, I guess it won't work /s
But here is a 660nm high power LED at least...
From its data sheet:
The LZ1-00R200 Deep Red LED emitter generates 900mW nominal output at 2.5W power dissipation in an extremely small package. The LZ1-00R200 LED emitter provides superior radiometric power in the wavelength range specifically required for plants’ chlorophyll a absorption.
... so there's a high chance there's a similar LED in the TendLite tbh.