In the US, most government (City, County, State) agencies enjoy a reasonably high level of sovereign immunity, so their inspections and approvals are basically worthless from a liability standpoint. As for signing/sealing the letter, as long as you are not practicing outside your area of expertise you are probably OK to sign individually; however, you need to understand a few things.....
1. In all states in the US, you are individually licensed, even though you work for a company that might have multiple engineers, holds itself out to the public as an engineering firm and has a certificate of authorization (a number). That CA number means nothing to an individual signing/sealing event, other than the fact that you might be a representative of that firm when using such.
2. Again, in the US, YOU hold the license to practice engineering. You can use that individually or as a representative of a company. If you sign the letter, you are "moonlighting". Is your company OK with that? Are you providing services individually that compete with your company? If so, you might want to explore the ethics of doing so.
3. Will your signing potentially expose your company to liability? If so, you might want to let them know what you are doing.
4. Are you willing to accept the liability for the deck? By signing/sealing you do so. Think about it.
Does your friend understand what he/she is asking? They are asking you to state, to the best of your professional ability, that the deck meets ALL local codes and standards; and is structurally adequate for its intended use. As cvg noted, read your state rules for engineers and if you are not capable of evaluating the structural efficacy of that deck, don't sign the letter.