If the reason your friend is installing a key logger is parental monitoring, there are better proactive programs for parental control rather than finding out after they have visited a dodgy site. Naomi Parental Control software (freeware)
http://www.radiance.m6.net/But, to answer your question directly, if your friend is happy there is no risk associated with installing this program, pause the standard shield provider so that it can be installed and enable standard shield after installation. You may then find that you need to add the programs executable file to the avast exclusions; add it to the exclusions lists of, Standard Shield, Customize, Advanced and Program Settings, Exclusions.