Either:
His old DVD player dose not suppor all formats of DVDs. (DVD+R, -R, etc.)
Or:
The DVDs you are using aren't compatible with that machine. Its not uncommon for different companies DVDs to work on certain machines and not others. Find a name that works, like memorex, verbatim, taiko yuden, etc. and keep in mind that different batches of DVDs can be different even if from the same company.