You don't actually have a job offer, you have a friend that has submitted your name to HR to be considered as a potential employee.
If you are seriously considering looking for another job, then put this one on the list, but only if you think your friend is a good judge of companies/work environments.
Among the things you don't know are:
- Will you be working near your friend? (this could be a plus or a minus)
- Do they have any interesting jobs?
- Do they have jobs in your desired salary range?
- Do they have jobs in a desirable work location?
- Long term prospects, future growth?
If your friend had the ability to give you a actual offer letter that would be different. What he has done is put you at step 0.5 of the process.
How you act is related to some of the other questions about interviewing for positions you don't intend to take.positions you don't intend to take.