T.R.
E., How wonderful for you to help your friend in such a thoughtful way.
My thought in the envelopes, if there isn't a way to get them pre-lined & embossed, is to have large colored stickers printed. Something in a neon pink or green, so she can clearly see the boundaries, and with 3-4 horizontal lines.
These can be affixed to the front of the envelope as a guide for her when addressing her envelopes. This can also serve the dual purpose of clearly identifying letters from her, much to the excitement of the recipient!
Another alternative, of course, is for her to type an address label on her computer & print it to stick on the envelope.
Hope this helps. T. =-)