A bail bond is a contract. You pay the bond company a certain amount, and they are obligated to pay the court the full amount of bail if the defendant doesn't show up.
Co-signing means you are just as responsible as the person who got bailed out. If he can't (or won't) pay, the bond company can come after you for the money.
Will they? It depends on the bail bond company. You'll probably do better by contacting them first, explaining that the defendant is in jail and asking if they'll wait until he gets out. If the payments get behind, they're less likely to work with you.
If the person bailed is back in jail usually the bail bond is exonerated and there is no liability (legal responsibility) for the co-signer to pay. Most bail bond agencies are very honest, but some are downright crooked. Be careful here. Only if the bail bond company legally had to pay the court the amount of the bond, and did in fact pay it, can the bail bond company then proceed against the co-signer for the amount called for in the contract the co-signer signed. If they do, then a good lawyer will know how to handle the situation. I recommend that the co-signer neither pay nor agree to pay the bail bond company anything without proof of an actual loss in the first place.
There is no quick practical answer. The legal answer is that you are just as liable as your friend in jail. Everything is written in the agreement which you and your friend signed. I doubt anyone ever reads this things and they are often called "Contracts of Adhesion," because the drafter wants everyone and everything to stick to it. You may ask the court to exonerate the bond since your friend is in jail. You usually can work with the bail bonding company unless they are rip-off specialists. If they don't have to pay on the bond, neither should you. If they are legitimate businessmen, they will work with you so as not to tarnish their local image.
This is not legal advice, bu rather an answer to a hypothetical question from someone I don't know.