First of all, link back credit will never been add in the footer if you didn't wish to add the credit link (Please disable / keep unchecked the option) in plugin's panel. It will not add any link back credit at anyway (neither write as html/as hidden link).. (not like any other plugin that still add the hidden link even you didn't want too).
The second, about "im_your_friend" function as you can see in the plugin source code, it is only generate XFN meta tags in header that will help us to add your blog / website into our blogroll list. XFN also was W3C standard that usefull for search engine like Google, Yahoo, etc.. to get relationship information between one site to another site. So, you don't have to worry about negative impact on SEO with Google because my plugin to help you increase your SEO and not vice versa.
Thank you and let me know if you have more questions & feedback.