syrup for 8 months is not indicated for any type of cough. Cough syrup is mainly used to suppress the cough for some time till the body recover and cough finishes. I don't think thus can be prescribed for so long. Did your friend drinking the cough syrup for addiction purpose or for some cough??
Another important thing is that many cough syrup has very high alcohol content in it. Because of using this type of cough syrup for long time can lead to some type of alcohol dependence
and when your friend suddenly stop taking that he starts having the withdrawal symptoms
. Excessive sweating
and shaky hands can be the feature of the alcohol withdrawal
which may be your friend is suffering from. You don not need to panic because it will start regressing after few days if it is because of withdrawal.
Thanks and take care.