Many software packages come with a EULA which uses claims such as "by opening this package you hereby agree to ... (some laws going far beyond classical copyright)".
Now, what happens if your friend opens the package and installs it on his computer, and I would use the software not in a way permitted by the EULA.
For example, I decompile someone's code which they prohibit by EULA.
Under what grounds can I be sued? I never agreed to the EULA so it's not contract violation.
It's a contract violation if you're under the EULA. It may be a contract of adhesion, but such "clickwrap" contracts been found to be acceptable and enforceable in software EULAs out of necessity. However, there may be some limits.
If you're not under the EULA, as you argue, then you lack a license to use the software at all and it's an outright copyright violation and/or a theft or misappropriation of the software.
Whether or not you can be sued depends in part on what you do with it — if you don't release the material or otherwise cause damages then there's not much to sue for...
Added for clarification: to answer the framing question, supposing neither contract or copyright applied, one could be sued in tort or in equity (i.e. for unjust enrichment).
Under the circumstances you describe, the friend who installed the software is unambiguously subject to the EULA.
The EULA presumably includes a provision prohibiting your friend from letting you use it, much less reverse engineer it.
Therefore, your friend is in violation of the license agreement and will be liable for any resulting damages. If you caused the damage, you may, depending on the circumstances, be liable in turn to indemnify your friend for the damages.
