It might be safe or it might not.
Depending on:-
the actual date of manufacture (may be a year or more than the date your friends bought it) the DOM will be stamped on the seat into the plastic - it might be a series of 'clocks' or a grid with 'pips' the last one will indicate the month/year made. If it's made in 2005 or earlier I'd be tipping it, personally.
on the condition of the straps (frayed/have they been washed/submersed in water),
the condition of the shell (take off the cover and look for any whitening of the plastic/cracks/EPS foam broken/missing),
how it was stored - if it was stored in a loft or garage or used in the car for the last 5 years I'd decline due to the extremes of temperature the plastic would have undergone potentionally weakening it,
whether or not you have the manual,
does it fit in your car/does it fit your child (e.g. not a booster for a 9m/head over seat shell/etc),
is it ECE R44/03 or ECE R44/04 or an earlier standard (03 was introduced in 1997),
and how long you plan to use it for.
If you want to use it for more than 1 year I'd start saving for a new one as most car seats have a 5-6 year shelf life. Car seats are, by law, stamped with a Do Not Use After date in the US and it can get you a ticket/fine if you are caught using one after this date as plastic degrades, fabric straps weaken and become frayed, safety standards move on and parts/manuals get lost/broken over time.
Sorry there's not an easy answer.