As you've been told, you can find books for teaching Spanish on SBS Bookstore, but you need to check the one in Angamos Av., next to the Scotiabank, a block away from the Icpna (intersection of Angamos and Arequipa).
If you want a piece of advice, I'm a Spanish as a second/foreign language teacher and I have used a wide range of materials available in Lima and Spain (and amazon.com).
I wouldn't recommend, as you've noticed, any material made for English speakers, which usually place their instructions in English, and even, some of them still try to work through translations. I agree with you on using a book that is all in Spanish, as they can keep the student immersed and help them learn faster.
The books that you can find in Lima (SBS), imported from Spain, and that I could find useful would be:
Español en Marcha 1-4: It's an affordable book with a communicative methodology. Good for a start.
Agencia ELE: It's a modern book for teaching adults. Expensive.
Avance 1-4: It's good for adults, but teenagers might get bored.
If you can easily buy on Amazon.com (or Amazon.es since it's cheaper there), I would recommend for your case "Gente Joven" (libro del alumno and cuaderno de ejercicios). It uses a task based approach, which works well with teenagers. Also, since your student is a francophone, he is going to be able to progress faster than an English speaker. You might need a new book every 30-40 hours of class.
I hope it helps, if you need any more advice, send me a message any time or email me at [email protected]
. I would be happy to help.