My friend said she took 2 pregnancy tests and they both came out "invalid" (not negative, not positive...no lines at all). So a few days later she tried another, and it was the same thing! I believe she was using the EPT brand. Why would this happen?

She should follow the instructions exactly.
She should take it in the morning when she first gets up and be sure to hold it straight and not to shake it up at all.

It either means
one. it may be too early to detect but possibly positive
two. the test was not done correctly
try doing the test first thing in the morning as it is more clearer at that time and read the instructions exactly
make sure it is left in for the right time

I have no clue. Out of all the test I've seen with my friends and just recently myself we always got a result. I took a dollar store pregnancy test as a joke because it came as one of the many gifts in a Bridal Shower Gift Bag. Then I took an EPT and got the +. You may want to call the company or get another test.