The following is all under the premise that there are no unknown factors that make it unlikely that the respective statement is misunderstood romantically, e. g., a large age gap.
Though you can reduce the risk that “ich habe dich lieb” is misunderstood romantically almost to zero by timing and intonation, this is a very difficult task even for a native speaker.
Of course, if you obviously are no native speaker, the other person will most probably instantly recognise the possibility that you meant the respective other thing – but unfortunately this goes in both directions. So, a phrase that bears a relatively low risk of being understood romantically if uttered by a native speaker like “Ich mag Dich sehr gerne“ is even more likely to be considered as a possible mistranslated romantical advance if uttered by no native speaker. (Similar problems appear with gestures by the way, e.g., most Germans do not know that holding hands has no romantic connotation in some cultures they (wrongly) perceive as rather conservative and reluctant towards body contact.)
So, if you really want to avoid an awkward situation at all costs, I would recommend to avoid unspecific statements of appreciation altogether and instead specifically thank her intensively for her help or something similar. However, this way less appreciation is conveyed – even more so, if your native language or culture bears the cliché of overly intensive thank-yous. So it might be wise to enter a little risk of misunderstanding here, especially since the fact that you are very good friends makes it even more likely that you can easily solve a misunderstanig or avoid it altogether by non-verbal communication. However, at the end of the day, you have to weigh the arguments and decide.
PS: The only way I came up with that avoids disambiguity is to explicitly mark the statement as non-romantic, e.g.:
Ich hab dich lieb (und das ist jetzt nicht romantisch gemeint).
However, this is most probably not a very good idea either, since it can lead to another bunch of awkward misunderstandings, like your friend thinking that you think that she has romantical interest in you, which you want to reject. It may be even hurtful, if your friend actually has a crush on you.